(a) Field of the Invention
The present invention relates to an apparatus and method of compensating for a frequency offset in an orthogonal frequency division multiple access system.
(b) Description of the Related Art
In general, wireless communication systems have been developed to transmit desired data using a predetermined frequency. The wireless communication systems are divided into a plurality of types of systems. Typical wireless communication systems are mobile communication systems, which are largely divided into synchronous mobile communication systems and asynchronous mobile communication systems.
A mobile communication system uses a method in which data is transmitted after multiplying a carrier signal of a predetermined frequency band by a code having orthogonality. In an IEEE 802.11 or 802.16 system, data is transmitted by using a technology according to an orthogonal frequency division multiplexing (OFDM) method or an orthogonal frequency division multiple access (OFDMA) method. In the system that uses the OFDM or OFDMA method, after data of a bit unit to be transmitted is converted into an OFDM symbol, the OFDM symbol is carried to a predetermined carrier signal and transmitted.
According to the structure of the OFDMA transmission system, the OFDMA transmission system is largely divided into a transmitter and a receiver. The transmitter converts raw data of a bit unit to be transmitted into a symbol, and carries the symbol to a high frequency so as to transmit it. The receiver receives the symbol that is transmitted by the transmitter and restores the raw data of a bit unit that is transmitted by the transmitter. The receiver of the OFDMA system estimates a frequency offset by using a preamble signal during a downlink, and compensates for a received signal on the basis of the estimated frequency offset.
In a wireless communication environment, such as the OFDMA system, the frequency offset occurs due to the relative difference of an oscillator between the transmitter and the receiver, that is, between a base station and a terminal, and a Doppler frequency that is generated when the terminal moves. In a system that uses a single frequency, the frequency offset reduces only a signal-to-noise ratio (SNR) of a received signal. However, in a system such as the OFDMA system that uses a multi-carrier, in addition to the reduction of the SNR of the received signal, inter-carrier interference (ICI) is caused to occur. Accordingly, research on synchronization to compensate a carrier frequency offset in a system using a multi-carrier as in the OFDMA system has been actively performed.
A method using a preamble, which is one of the methods of compensating for a frequency offset in an OFDMA system in the related art, estimates a frequency offset by using a repetition characteristic of a guard interval. In this method, a range of a frequency offset that can be estimated is limited to less than half of a subcarrier frequency interval. As a result, it is needed to provide a separate structure for estimating a frequency offset in a frequency offset range that is not less than half of the subcarrier frequency interval.
That is, in an OFDMA system such as an IEEE 802.16e or WiBro OFDMA system that uses a downlink preamble, a frequency is estimated by using a characteristic that the preamble signal is repeated in a time domain in order to compensate for a fractional frequency offset and an integer frequency offset. At this time, if the number of times the preamble signal is repeated in the time domain is M, since M is 3 in the WiBro OFDMA system, the frequency offset needs to be compensated by 1.5 times as much as the subcarrier frequency interval. However, the preamble signal is not repeated in an actual time domain three times.
Accordingly, in the WiBro OFDMA system, a frequency is estimated by using a repetitive characteristic in a guard interval of the preamble instead of a repetitive pattern of the preamble signal. Since a range of a frequency offset that can be estimated is limited to less than half of a subcarrier frequency interval, it is needed to provide a separate structure that estimates a frequency offset in a frequency offset range that is not less than half of the subcarrier frequency interval.
Examples of a method of compensating for a frequency offset in the OFDMA system according to the related art include methods of estimating and compensating for fractional and integer frequency offsets. However, a method is not described in which the receiver of the OFDMA system efficiently compensates for fractional and integer frequency offsets after efficiently estimating the fractional and integer frequency offsets.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.